In general, when a pump is suddenly stopped or a valve closes suddenly in a water pipeline system, a transient condition in which an amount of flow and a hydraulic pressure are dramatically changed occurs. This phenomenon is referred to as water hammer or fluid hammer.
As a result of water hammer, pressure inside a pipe is dramatically increased or is decreased to be lower than saturated vapor pressure such that vapor occurs, and after that, in the process of column separation and return, the pipe may be broken or damaged by a shock wave.
For example, as shown in FIG. 1, a water pipeline system includes: a water supply pump 2 supplying water from an inlet 1 to one direction; a main pipe P with the water being transferred therethrough; and an outlet 3 for discharging the water transferred from the main pipe P.
Further, the main pipe P may be provided not only with a check valve 4 for preventing back flow, but also with a flexible joint for preventing vibration and a shut-off valve for controlling the water flowing through the outlet 3.
Here, when the water supply pump 2 is stopped or the shut-off valve is quickly closed, velocity of flow is dramatically changed in the main pipe P between the inlet 1 and the outlet 3, and thereby water hammer occurs, damaging the main pipe P or the water supply pump 2.
For the above reason, as shown in FIGS. 2 and 3, according to a document of related art, such as Korean Patent application publication No. 10-2013-0093299, a check valve is configured such that a shock-absorbing damper 50 is connected to a rotation shaft 20 of the disc 30, whereby in the case of quick closing of the valve, noise and vibration caused by a collision between a disc 30 and a valve body (for example, a valve seat surface), and water hammer caused by dramatic change in velocity of flow are prevented.
Thereby, as shown in FIG. 2, even when the pump is stopped and the disc 30 moves down by weight of a balance weight 40, the disc 30 is slowly closed, that is, a slow closing function is provided by the shock-absorbing damper 50 constituted by a hydraulic cylinder, and the like.
However, a conventional check valve described above provides the slow closing function, in which the disc 30 is slowly closed, by absorbing shock generated when the disc is closed, through the only single shock-absorbing damper 50. Accordingly, the conventional check valve is problematic in that it is impossible to provide a perfect slow closing function by using the single shock-absorbing damper 50.
The conventional check valve is further problematic in that the slow closing, in which the disc 30 is slowly close, is performed to prevent noise, vibration, water hammer, and the like caused by the quick closing, but during the slow closing, it is impossible to perform a function unique to the check valve, which prevents a fluid from flowing backward.
In other words, if it takes long for the disc 30 to move down to close a passage, during the time, a massive amount of fluid (namely, water) flows backward, and accordingly it is impossible for the check valve to perform its function. Moreover, the back flow may apply a pressure to the stopped water supply pump 2, or may cause a problem by making the water supply pump 2 rotate reversely.
Accordingly, when quick closing of a check valve is performed, noise, vibration, and water hammer occur; and when the slow closing is performed to solve this problem, the back flow of the fluid is increased, so both the quick closing and the slow closing are problematic.
In particular, when the main pipe P is provided with an air chamber 3 to prevent water hammer, the air chamber 3 is expensive, and may accelerate the quick closing of the check valve by a hydraulic pressure caused when make-up water stored in the air chamber 3 is supplied to the main pipe P when the pump is suddenly stopped.